In electrophotographic printers commonly in use today, a photoconductive insulating member is typically charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure discharges the photoconductive insulating surface in exposed or background areas and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with developing powder referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating area to form a powder image on the photoconductive area. This image is subsequently transferred to a support surface, such as copy paper, to which it is permanently affixed by heating or by the application of pressure. Following transfer of the toner image to a support surface, the photoconductive insulating member is cleaned of any residual toner that may remain thereon in preparation for the next imaging cycle.
One of the more conventional approaches to fixing the toner image is through the use of heat and pressure by passing the print substrate containing the unfused toner images between a pair of opposed roller members at least one of which is internally heated. During this procedure, the temperature of the toner material is elevated to a temperature at which the toner material coalesces and becomes tacky. This heating causes the toner to flow to some extent into the fibers or pores of the support member. Thereafter, as the toner material cools, solidification of the toner material causes the toner material to become bonded to the support member. Typical of such fusing devices are two roll systems wherein the fuser roll is coated with an adhesive material such as a silicone rubber or other low surface energy elastomers. The silicone rubbers that can be used as the surface of the fuser member include room temperature vulcanizable silicones, referred to as RTV silicones, liquid injection moldable or extrudable silicone rubbers, and high temperature vulcanizable silicones referred to as HTV silicones. Other known suitable materials for the surface of the fuser roll include those sold under the trade names VITON and TEFLON, the latter being a polymer as opposed to an elastomer.
During the fusing process and despite the use of low surface energy materials as the fuser roll surface, there is a tendency for the copy print substrate to remain tacked to the fuser roll after passing through the nip between the fuser roll and the pressure roll. When this happens, the tacked print substrate does not follow the normal substrate path but rather continues in an arcuate path around the fuser roll, eventually resulting in a paper jam which will require operator involvement to remove the jammed paper before any subsequent imaging cycle can proceed. As a result it has been common practice to use one or more techniques to ensure that the print substrate is stripped from the fuser roll downstream of the fuser nip. One of the common approaches has been the use of a stripper finger or a plurality of stripper fingers placed in contact with the fuser roll to strip the print substrate from the fuser roll. While satisfactory in many respects, this suffers from difficulties with respect to both fuser roll life and print quality. To ensure an acceptable level of stripping it is frequently necessary to load such a stripper finger against the fuser roll with such a force and at such an attack angle that there is a tendency to peel the silicone rubber off the fuser roll, thereby damaging the roll to such an extent that it can no longer function as a fuser roll. In addition, since the finger comes in contact with the surface of the print substrate which has hot, just fused toner image thereon, there is a tendency for the stripper finger to scrape toner from the print substrate thereby creating a copy quality defect in the form of a line which may be the width of the stripper finger. Furthermore, while a stripper finger may only slightly deform the toner this may create a defect in the form of a stripe of higher gloss than the rest of the print. It has also been found that stripper fingers typically made of high energy materials become contaminated with toner on the side in contact with the fuser roll, eventually resulting in the stripper finger lifting off the fuser roll and resulting in paper jams.
Another common problem associated with stripper fingers as they have been heretofore used is "lead edge nicks" in the support material, such as sheets of paper. Lead edge nicks occur when a substrate, such as a sheet of paper, strikes a stripper member which is too thick at the tip. Number 20 paper, for example, is 0.09 mm thick with a substantially square edge, while an uncoated stripper member is typically 0.1 mm thick and also square edged. If the edge of the stripper member is angled 15.degree. toward a tangent of a 75 mm diameter fuser roll, a finger thickness up to 0.14 mm is acceptable to avoid failure of the stripper member to strip the copy off the fuser roll. However, lead edge nicks are persistent if the tip is larger than 0.025 mm thick if the tip or the paper corner is square-edged, or 0.075 mm thick if rounded. Coatings on the stripper member may add up to 0.075 mm to the finger thickness.
As a result of the difficulties associated with stripper fingers, use has been taken in many instances of air stripping systems. While satisfactory in many respects, the air stripping systems are typically very expensive, involving elaborate air delivery systems.
U.S. Pat. No. 4,687,696 to Satoji describes a finger strip for separating sheets of paper from a fuser roll in a copying machine which is made of a heat resistant resin and has at least a tip portion coated to a thickness of about 40 angstroms to 1 micron of fluorinated polyether polymer to improve lubricity and add anti-stickiness. High adhesion strength between the coating and the finger help to eliminate the problem of poor separation and jamming of paper.
U.S. Pat. No. 4,929,983 to Barton et al. describes a stripper for separating a print substrate from a fuser roll. The stripper has a substantially flat, thin, resiliently flexible finger-like member with a raised dimple-like bump for contacting the print substrate when it is stripped from the fuser member. The finger-like member is coated on both sides with a smooth low surface energy film.
U.S. Pat. No. 4,796,880 to Tamary discloses a skive for removing a copy sheet from a hot fuser roller. The skive engages the fuser roller at a small acute angle so that a sheet will be deflected from the roller along the desired paper path. An anti-gouge stiffener is provided adjacent the finger, which is engageable by the finger to prevent it from flexing in the wrong direction.
It is an object of the present invention to provide a stripper finger for use with a fuser roll in an electrophotographic apparatus which avoids many of the problems of prior art devices, such as lead-edge nicks on sheets coming off the fuser roll, bending of the stripper finger, and damage to the fuser roll.
It is another object of the present invention to provide such a stripper finger which is simple to manufacture from inexpensive materials using commonly-available techniques.
Other objects will appear hereinafter.